That would be the vi editor, of course. For some reason the folks over at Hacker News have been posting some old interviews with Bill Joy about the vi editor. In Bill Joy’s greatest gift to man—the vi editor, Ashlee Vance of The Register posits that even more than BSD Unix, NFS, or the UltraSPARC, Joy’s greatest accomplishment is vi. Certainly there’s a lot of people who think so. The rest of the article is an interview of Joy that explores the history of vi a bit.
Steve Kirkendall points to a 1984 Interview with Bill Joy from the old Unix Review magazine that discusses the history of vi in a bit more detail. Both articles are interesting and worth a read.
I used vi (and vim) for many years and still have warm feelings for it. It’s a great editor and along with Emacs (and arguably TextMate) one of the only 2 or 3 editors that serious developers use. I switched to Emacs when I started using Lisp and Scheme seriously (as described here and here) and wouldn’t change back but I still have a lot of respect for vi.